On vehicles with gas-fuelled engines, the gaseous fuel is loaded and compressed in at least one cylinder or container. According to safety standards in the sector, each gas cylinder in a vehicle is equipped with safety means to prevent, or at least reduce the chance of, an explosion due to an abnormal increase in the internal pressure, the presence of fire and/or a temperature increase beyond a predetermined threshold. Security units are used in association with compressed gas cylinders to prevent excessive temperature increases that lead to increases in the internal pressure of the cylinder.
Known heat safety devices are traditionally placed along a gas venting duct from the cylinder. They typically include a heat sensitive element confined in a respective seat or chamber in a solid body in association with an ejection piston. The heat sensitive element keeps the duct closed unless and until it reaches a predetermined safety temperature threshold upon which it opens the vent for emptying the cylinder.
In certain embodiments, the heat sensitive element is a solid state alloy that melts at the predetermined safety temperature. The element is then pushed, at least partially, outside through an ejection port by the associated piston so as to open the venting duct. In other embodiments, the heat sensitive element is in liquid or paste form and contained in a casing configured to break upon reaching the predetermined temperature.
It is important that the safety devices be reliable and sensitive enough to work properly when impinged by an abnormal rise in temperature.
Several solutions have been developed to increase the reliability and sensitivity of the safety devices. In one method, a small amount of eutectic material is placed in the proximity of the external-environment. Tests have shown that the sensitivity and reliability of the safety device increases as the amount of eutectic material is decreased and the proximity between the eutectic material and the heat source, such as the environment outside the device, increases. One problem with this method is that these devices are difficult to manufacture and assemble. Any mistakes/faults in the manufacturing and/or assembly can adversely affect the operation of the device itself.
In other techniques, device sensitivity is increased by using a liquid heat-sensitive element that is contained in a glass casing configured to break at a predetermined temperature. In these devices, when the predetermined temperature is reached, the casing breaks and the gas is immediately released. Such devices are described in patents German Patent No. 19911530, European Patent No. 0590235 and European Patent No. 0314577.
One problem encountered in the above devices involves the glass casing, which is typically made in the shape of a bulb with a thin elongated neck. This glass casing is known to break for reasons unrelated to the temperature, such as vibrations caused by the operation of the vehicle.
When the glass casing breaks before reaching the predetermined temperature then if and when the predetermined temperature is reached, the safety device fails to work.
Another concern is that the device offers little resistance where the force that the high pressure gas imparts to the piston is great.
There is a need for inexpensive easily manufactured safety devices that reduce the time required for activation. This reduced time makes them both more sensitive and reliable.